The present invention relates to a method of optimising the functioning of a submerged arc furnace for producing molten metal.
It is known how to use submerged arc furnaces (SAFs) for the production of ferro-alloys (Fexe2x80x94Mn, Fexe2x80x94Cr, etc.) and pig irons by the reduction and melting of charges of already partially pre-reduced ore, particularly by using coke as a reducing agent. It is also known how to use these SAFs for the reduction and melting of metallic residuary products, particularly metal powders which are charged in the form of fines, granules or pellets. The reduction and melting methods used in these SAFs are generally distinguished by a considerable production of slag; the mass of slag is often comparable with or even greater than the mass of metal. The bath of metal is consequently covered by a layer of molten slag of considerable depth (about 0.4 m to 1.50 m), which represents a charge of about 1 to 3.75 tonnes of slag per square metre of the bath. The electrodes of the SAF are located in the central zone of the furnace, while the charge is loaded into the peripheral zone, i.e. between the central zone with the electrodes and the furnace wall.
In the SAF, the heat energy required for melting the metallic products is generated by the conduction of the current through the molten slag. Consequently, there is no actual plasma arc (or free arc) set up between the electrodes and the bath of metal. The electrical power developed in an SAF is therefore only from 0.2 to 0.5 MW per m2 of crucible surface area, which is a very low power compared with one of about 2 MW per m2 of crucible surface area developed in free arc furnaces. Since the energy requirements for the reduction and melting of ferro-alloys or metallic residuary products are, on the contrary, very high, the result is that the productivity of SAFs currently leaves a lot to be desired.
It is therefore an object of the present invention to increase the productivity of a SAF.
The method according to the invention makes it possible to optimise the functioning of a submerged arc furnace for the production of molten metal. This electric furnace incorporates at least one electrode and contains a bath of molten metal covered with a thick layer of molten slag having a mass per unit area of at least 1 t/m2. According to an important aspect of the invention, the slag is made to foam locally around the electrode so as to create around this electrode a local layer of foaming slag in which the density of the slag is at least 50 per cent lower than in the rest of the furnace. If the furnace has several electrodes, the slag is preferably made to foam locally around all the electrodes in the furnace.
The method according to the invention offers the possibility of optimising the functioning of an electric SAF containing a large amount of molten slag, and particularly of increasing its productivity. In effect, the creation of a local layer of foaming slag changes the way in which the electric energy passes into the bath. Conduction of the electric current through the resistive molten slag is at least partially replaced by a xe2x80x9cplasmaxe2x80x9d arc formed in a gaseous medium, even if this medium also includes a certain proportion of molten slag. It is possible in this way to improve the characteristics of the arc, i.e. the arc voltage and the length of the arc. The electric field in plasma mode immersed in a foaming slag is at least two to four times larger than in the resistive mode (conduction in the molten slag). As a result, there is an appreciable increase in the power of the SAF. Since the power is higher, it is possible to reduce the melting time and hence increase the productivity. Besides, the conditions for the production of molten metal in foaming slag are less severe than in molten slag. That is the reason for the lower consumption of the electrode or electrodes in the local layer of foaming slag.
The electric furnace may contain a bath of molten metal covered with a thick layer of molten slag having a thickness from 0.4 to 1.5 m. Preferably, the local layer of foaming slag surrounding the electrode comprises at least 50 per cent of gas, and optimally at least 80 per cent of gas.
Advantageously, the local layer of foaming slag is formed by the addition of at least one carbon-containing fuel and/or at least one oxidant, in or on said layer of slag and/or said bath of molten metal. The reaction of the carbon-containing fuel with the oxygen contained in the bath of molten metal produces CO, which causes the slag to foam. In addition, the combustion of the carbon-containing fuel provides an input of energy which is then added to the heat energy of electrical origin for the reduction and the melting. By injecting the gas containing oxygen into the top third of the layer of slag, post-combustion of the CO can be achieved, i.e. a reaction which also contributes to the input of heat energy.
According to a preferred mode of execution, the electric furnace comprises at least three electrodes located in the centre of the furnace. A local layer of foaming slag is then created between the three electrodes. In other words, the local layer of foaming slag surrounds each electrode and extends between the electrodes at the centre of the furnace.
Preferably, the raw materials are added mainly at the centre of the electric furnace.
According to a first mode of execution, the raw materials are added in the form of fines injected into the lower part of the layer of slag and/or into the bath of molten metal. It is, for example, possible to use this method to inject fines of pre-reduced iron ore.
According to a second mode of execution, the raw materials are added in the form of pellets or bricks. This makes it possible to introduce into the electric furnace raw materials that cannot be injected, by agglomerating them for example into pellets or bricks with a high enough density to penetrate the layer of slag.
Other special features and characteristics of the invention will emerge from the detailed description of an advantageous mode of execution given below, as an illustrative example, making reference to the appended drawing. This shows: